The invention relates to a hydraulic device for back and forth movement of a machine part according to the preamble of claim 1. The invention further relates to a clamping unit of an injection molding machine fitted with such a hydraulic device.
Various embodiments of conventional hydraulic devices are known that have a functional separation which is so configured that, on one hand, closing and opening takes place with little force and at high speed, and, on the other hand, the final approach and locking of the half-molds of the injection molding tool takes place with great force and at slight speed.
DE 6605115 U1 discloses the arrangement of a first piston as high-speed piston, provided on one end of a piston rod and sized of comparably small diameter, and of a second piston as working piston, provided on the other end of the piston rod and sized with comparably significantly greater diameter, whereby each of the pistons is guided in separate cylinders. In this way, a small area of the high-speed piston can be acted upon by pressure medium, and a significantly greater area of the working piston can be acted upon by pressure medium.
The documents DE 9013791 U1 and FR 2739149 disclose a guidance of the working piston in a working cylinder and an arrangement of the high-speed cylinder inside the piston rod of the working cylinder. In order to generate a greatest possible clamping force, DE 9013791 describes to size the effective area of the working piston large enough, whereas FR 2739149 describes to pressurize the chamber for the working piston as well as the chamber for the high-speed piston in a suitable manner so that the effective area of the high-speed piston participates in the development of the locking force.
Further known are hydraulic devices (DE 9014885 U1, DE 1805938 A1), in which the piston rods and the cylinders of high-speed piston, on one hand, and working piston, on the other hand, move within one another, whereby the high-speed piston and the working piston can be hydraulically locked together for building up the clamping force and for locking, while the machine part is locked by subjecting only the working piston to pressure medium.
DE 8533535 U1 discloses a hydraulic device for back and forth movement as well as locking of a machine part, in particular for opening, closing and clamping the half-molds of an injection molding tool of an injection molding machine. Provided in a cylinder is a first, large pressure space with a pressure medium in which a primary piston is axially movable having several piston rods that are coupled to the machine part to be moved. The primary piston includes several openings for allowing the pressure medium to flow from one side of the primary piston to the other side so that the primary piston is able to float in the pressure medium. Disposed in the primary piston for axial movement therein is a small lockable secondary piston for operating a flap valve to clear or close the openings in the primary piston. When closed, floating of the primary piston is prevented.
This known apparatus operates essentially as follows:
The areas subject to the pressure medium are so suited to one another that the primary piston is floatingly pushed to the right, when the flap valve is open and pressure medium is fed to the pressure space, and the tool of the injection molding machine is closed. Only little pressure medium is required and the movement is executed at great speed but with little force. The half-molds of the injection molding tool are clamped together by actuating the small secondary piston, disposed in the primary piston, to operate the flap valve and to inhibit the floating of the primary piston. As a consequence, pressure builds up only on the side of the primary piston, distal to the piston rods, as the injection molding tool travels a final distance at slight speed but great force and is subsequently clamped. The injection molding tool is opened by supplying pressure medium to the auxiliary cylinders, which accommodate the piston rods of the primary piston, and the flap valve is opened. The primary piston can now move floatingly to the left, and the injection molding tool opens.
A drawback of this apparatus is the relative complexity by which the secondary piston is disposed inside the primary piston for operating the flap valve, and the fact that the hydraulic lines for operating the secondary piston must be so routed through the cylinder and connected to the pressure space of the secondary piston inside the primary piston as to follow the back and forth movement of the primary piston in the primary cylinder. A further drawback is the need for separate auxiliary cylinders provided for the piston rods of the primary piston and mounted to the primary cylinder, as well as the provision of a further auxiliary cylinder in midsection for a compensating piston so that the hydraulic device is overall significantly longer than the length of the primary cylinder.